I've found that on all of the machines that I was having the issue on, the Sharing addin was somehow disabled. How it got there is beyond me because nobody here would have gotten the idea to manually disable it. In one case I had to go set the registry key (which I can't remember exactly right now) to 3 by hand as no plugins were listed as disabled.

My user's Outlook doesn't crash when the debug issue occurs. The debug error always comes up when she goes to send an email. She hits ignore and the message sends, however it's pretty annoying having to deal with that debug popping up.

I had one user that didn't have any options for disabled addins, but on checking his registry the value was 2. I don't honestly know what load mode that's supposed to define, but it's not right. Changing to 3 worked.

registry set to 3 and enabled plugin/ still not working

I have a user with the debug window popping up on send who shows that the registry was already set at 3, and the Zimbra Client Extension was already enabled. Still this behavior occurs. Has anyone else seen/dealt with this?